The present invention relates to microwave probes for tissue ablation and in particular to a microwave antenna providing improved localization during tissue heating.
Microwave ablation may be used to treat tumors, for example in the liver, in patients who are not eligible for surgical removal of the tumor. In such microwave ablation, a coaxial microwave antenna is inserted into the tissue to the point of the tumor to conduct microwave energy to that location. Tissue ablation is caused by depositing energy in the region surrounding the antenna and its conversion to heat. Energy deposited into the tissue is often characterized by the Specific Absorption Rate (SAR). The SAR pattern, and thus the size and shape of the ablation region, is a function of the microwave power, the duration of the application of power, and the design of the antenna. Microwave ablation does not require a separate ground pad attached to the patient, and thus may be distinguished from ablation at lower frequencies.
Currently, microwave ablation can effectively treat tumors only less than about two centimeters in diameter. This is because microwave power to the antenna must be limited to prevent excessive heating along a “tail” away from the tumor and along the length of the antenna. During percutaneous treatment, this tail of heating may damage healthy tissue and burn the skin.
This heating in the tail along the antenna results from a number of effects including the SAR pattern of the antenna, resistive heating of the shield of the coaxial antenna structure, and thermal conduction of heat along the metallic conductors of the antenna from the tumor site. Different types of antennas have been developed to prevent or reduce this heating tail including designs that use gaps and ring structure attached to the center and/or outer antenna conductor to provide capacitive and inductive elements to form resonant traps blocking current flow on the outer conductor.